All woods are divided into two major classes on the basis of the type of tree from which they are cut. Hardwoods are from broad-leaved, deciduous trees. Softwoods are from conifers, which have needle- or scale like leaves, which are, with few exceptions, evergreens. These terms do not refer to the relative hardnesses of the woods in these two classes.
Both hardwood and softwood logs are cut into planks and boards in a saw mill. When rough dimension lumber is cut from a fleshly cut log it may have a moisture content from about 35 to more than about 70 percent water. Before this wood is ready for use it must be seasoned.
Wood is seasoned either by air drying, kiln drying, or a combination of air and kiln drying. Air drying may be considered to give superior quality, but it requires more time, is expensive, and is indefinite. Even lumber that has air dried for a full year may still have a moisture content of about 15 to about 20% or greater. Numerous tests made at the U.S. Forest Service Products Laboratory have failed to show any superiority in air-dried wood when kiln drying was performed properly. Standard schedules for drying wood are published in the U.S. Forest Service Dry Kiln Manual. The drying schedule will vary depending upon the variety of wood, the initial moisture content, and the thickness of the lumber.
These standard drying schedules recognize that the strength of the wood is directly affected by the conditions under which it is dried. More specifically, removing moisture from the wood too quickly can result in cracking or embrittlement of the wood. Drying conditions that are optimal for one variety of wood may cause severe damage to another variety. Therefore, it is important that a dry kiln be capable of accurately creating and controlling wood drying conditions.
Kiln conditions that are most important to the process of drying wood include the air temperature, circulation, and humidity. Accurate control of these conditions minimizes cracks and other defects in the wood while minimizing the time and energy required to dry a batch of lumber.
An exemplary drying schedule for hardwood lumber is that of Red Oak lumber having a thickness between 1 inch and 1.5 inches, obtained from the Dry Kiln Operators Manual, United States Department of Agriculture, Agriculture Handbook No. 188 (August 1991), as set out below:
______________________________________ Moisture Content Dry Bulb Wet-Bulb Wet Bulb (Percent) Temp. (.degree.F.) Depression (.degree.F.) Temp. (.degree.F.) ______________________________________ &gt;50 110 4 106 50 110 5 105 40 110 8 102 35 110 14 96 30 120 30 90 25 130 40 90 20 140 50 90 15 180 50 130 ______________________________________
Most dry kilns currently in use are similar to metal buildings constructed on concrete slabs with heaters, fans, and air-duct systems for delivering warm air to the wood. After the air passes over the wood, it is drawn into an intake vent. The air is then reheated and recirculated back to the delivery vent of the air-duct system. After drying over a period of time, the system is shut down, the kiln is opened, and the wood is sampled. If the water content of the wood is decreasing too rapidly or too slowly, the air temperature and/or air circulation rate can be adjusted. The kiln is resealed, the air is reheated, and the drying process is continued under the adjusted conditions for another period of time. This sequence is repeated until the lumber has been sufficiently seasoned. Each time a kiln is opened, the relatively cool outside air enters the kiln. This is not only a waste of thermal energy and time, but is detrimental to the drying of the wood. It would therefore be desirable to monitor and control the drying of wood lumber without opening the kiln.
Because the air circulation fans, electrical wiring, and electrical connections are housed within the kiln, they are continually exposed to the wet, acidic moisture drawn from the wood. The motors which drive the fans are quick to corrode and require frequent replacement. Aside from the cost of replacing the fan motors, it is inefficient for the kiln to be shut down and opened in order to install a new motor. Because of this arrangement, poor air circulation may go undetected or, if detected, be tolerated for extended periods of time causing uneven drying of the wood. However, replacing the motor immediately means opening up the kiln one more time. It would therefore be desirable to be able to monitor, maintain, and replace fan motors while the kiln remains in operation.
Furthermore, because conventional kilns are large, stationary equipment, lumber must be hauled great distances at significant expense. It is particularly unfortunate that large amounts of diseased or infested forestation can not or should not be transported beyond the boundaries of the affected areas to a kiln without spreading the problem. The result is that the affected trees may be burned on site to prevent the problem from spreading. It would therefore be desirable to have a kiln that could be transported into the affected areas to season the wood. The elevated temperatures of the kiln would kill the bugs or bacteria responsible for the problem so that the seasoned wood could be taken from the area for commercial purposes.